1. Field
This disclosure relates generally to a system and method to effectively support Voice Call Continuity (VCC) for Voice-Over-IP (VoIP) emergency calls.
2. Background
Voice Call Continuity (VCC) is a feature that is being standardized by the Third Generation Partnership Project (3GPP) and by the Third Generation Partnership Project 2 (3GPP2). A definition of VCC has been produced by 3GPP in 3GPP TS 23.206 (“Voice Call Continuity between CS and IMS; Stage 2”) which is a publicly available document. A definition of VCC has also been produced by 3GPP2 in 3GPP2 X.P0042-001-0 (“Voice Call Continuity between IMS and Circuit Switched Systems—Stage 2”) which is also a publicly available document. Both definitions of VCC are very similar and support continuity of a voice call from a wireless terminal to some other device (wireless or non-wireless) when the wireless terminal switches between using a wireless access that supports circuit mode and a wireless access that supports VoIP. In particular, use of VCC avoids having to release a call (e.g., a circuit mode call) and re-establish a call (e.g., using VoIP) when the user switches access which would cause significant delay and disturbance to the users involved in the call and might result in an inability to re-establish the call.
Specific examples of wireless access networks that support circuit mode, sometimes referred to as the circuit switched (CS) domain, include the Global System for Mobile Communications (GSM), Universal Mobile Telecommunication System (UMTS) using wideband code division multiple access (W-CDMA) and cdma2000 1X. Examples of wireless access networks that support VoIP include UMTS W-CDMA, cdma2000 1xEV-DO (Evolution Data Optimized) and various wireless LAN (WLAN) and WiMax networks. In some cases, the same access network (e.g. UMTS W-CDMA) can support both circuit mode and VoIP, although the user's wireless terminal would be required to use just one of them (at any one time) for a particular call.
When a wireless user loses coverage for a particular wireless access network and needs to make use of another access network, it is sometimes possible to handover an ongoing call from one access network to the other without disruption of service provided the type of access (circuit mode or VoIP) remains unchanged. When the type of access needs to change, however (e.g., because the new access network does not support the previously used access type), it will normally release the call and re-establish it again using the new access type. VCC is a capability that enables handover between VoIP and circuit mode without such a disruption.
VCC is currently defined by both 3GPP and 3GPP2 for normal non-emergency calls and may not explicitly support emergency calls originated using either circuit mode or VoIP. This is because the solution being defined for VCC in both 3GPP and 3GPP2 is incompatible with the solutions defined and being defined to support circuit mode and VoIP emergency calls. The main source of this incompatibility is that the solutions to support emergency calls rely on support from the network, known as the visited network, that is currently serving the wireless user, whereas the solutions for VCC require VCC support in the user's home network even when different from the visited network.
The lack of VCC support for emergency calls means that such calls will have to be released and re-originated if a wireless user needs to change between use of circuit mode and use of VoIP in different access networks (or in the same access network)—e.g., because the user has moved or the current access network is congested or subject to some other anomalous condition. This may be particularly disadvantageous because if the user re-originates the call, the call may not go through to the same Public Safety Answering Point (PSAP) operator (i.e., to the same person) as before. In addition, the PSAP operator may not be able to re-originate the call if a callback number was not provided when the call was first established—e.g. if the wireless terminal is not properly authorized in the original access network. There would thus be an advantage to extending the capability of VCC to support emergency calls.